Gate valves have been used for many years to prevent and permit the flow of fluids, such as oil, in pipelines. With today's stringent safety requirements, many of these gate valves contain fail-safe mechanisms so that the valve will close should actuator problems occur. A fail-safe type valve is one that must close automatically at a predetermined minimum line pressure without the aid of a spring. See for example U.S. Pat. No. 3,765,642, issued Oct. 16, 1973, to Norman A. Nelson; U.S. Pat. No. Re. 30,115, reissued Oct. 16, 1979 to David P. Herd, et al; U.S. Pat. No. 3,378,224, issued Apr. 16, 1968 to W. G. Boyle; U.S. Pat. No. 3,379,405, issued Apr. 23, 1968 to P. J. Natho and U.S. Pat. No. 3,958,592, issued May 25, 1976 to Richard E. Wells, et al.
For a gate valve to be fail-safe at any given line pressure, there usually exists a means of communicating line pressure into the valve body cavity. Pressure within the body cavity will then act upon the area of the valve shaft and produce an upward (closing) force greater than the friction forces developed between the gate and seats. However, many gate valves known in the prior art are directed to include upstream seats which are effective to exclude line pressure from the valve housing during normal operation. In addition, many of these also are directed to relieving valve housing pressure when it becomes excessive by bleeding the fluid in the valve housing back into the line. See for example U.S. Pat. No. 2,985,189, issued May 23, 1961 to S. R. Dickinson; U.S. Pat. No. 2,985,422, issued May 23, 1961 to C. E. Anderson, et al; U.S. Pat. No. 3,307,826, issued May 7, 1967 to W. E. Lowrey; U.S. Pat. No. 3,559,948, issued Feb. 2, 1971 to Marvin H. Grove; U.S. Pat. No. 3,834,664, issued Sept. 10, 1974 to Eulas R. Atkinson; and U.S. Pat. No. 4,161,309, issued July 17, 1979 to Thomas A. Klyce. For example, Dickinson disclosed a venting system for a valve body wherein the sealing ring is positioned in an annular groove having a varying cross-sectional area. This permits the sealing ring to move to a position to permit leakage of fluid from the valve body to the conduit. Klyce shows a gate valve in which upstream pressure is continuously applied across a seat ring to a valve housing by expansion of an o-ring on the upstream seat to a non-sealing position.
Some of the prior art uses a design which works with flowing gas, or when there is at least a small percentage of gas present within the flowing medium or fluid. In some of the prior art, with gas as the flowing medium or with a small percentage of gas present, the gate will move (i) from an open position to a position wherein pressure communication between the body cavity and the upstream side is lost prior to the gate fully closing and (ii) will continue to move from the position of (i) to a fully closed position by the expansion of the gas within the valve body cavity. However, when the flowing medium is only liquid, the valve would remain in a semi-closed position without the aid of a spring or other mechanism because of the incompressibility of liquids. Accordingly, insufficient force would be produced to overcome the friction between the gate and the seats to fully close the valve.
It is an object of the present invention to teach an apparatus that renders a gate valve fail-safe for liquids or gases even in the absence of the aid of a spring.
It is a further object of the present invention to teach apparatus to render a gate valve in liquid or gas service fail-safe without the aid of a spring while retaining a bidirectional design.
It is yet another object of the present invention to teach apparatus which renders a gate valve in liquid or gas service fail-safe without the aid of a spring while avoiding a pressure lock situation.